Non-volatile memory devices are an extremely important component in electronic systems. FLASH is the major non-volatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Disadvantages of FLASH memory include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the scaling of the gate oxide is restricted by the tunneling barrier seen by the electrons. Hence, FLASH memory is limited in the dimensions to which it can be scaled.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is the MRAM cell. To be commercially practical, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the nonvolatile memory state, the repeatability of the read/write cycles, and the memory element-to-element switching field uniformity are three of the most important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive differences in the MRAM device between the two states. The magnetic fields for writing are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
As the lateral dimension of previously known MRAM devices decrease, three problems may occur. First, the switching field increases for a given shape and film thickness, requiring a larger magnetic field to switch. Second, the total switching volume is reduced so that the energy barrier for reversal decreases. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device and unintended reversals can occur due to thermo fluctuations (superparamagnetism) if the barrier is too small. A major problem with having a small energy barrier is that it becomes extremely difficult to selectively switch one MRAM device in an array. Selectability allows switching without inadvertently switching other MRAM devices. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the MRAM device decreases in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.
A novel method of writing to MRAM cells has been disclosed in U.S. Pat. No. 6,545,906 comprising a method to switch a scalable magnetoresistive memory cell including the steps of providing a magnetoresistive memory device sandwiched between a word line and a digit line so that current waveforms can be applied to the word and digit lines at various times to cause a magnetic field flux to rotate the effective magnetic moment vector of the device by approximately 180°. This method provides two different modes of state switching: a toggle write mode where the state of the bit is changed or toggled each time two field pulses of both the same polarity are applied, and a direct write mode where the state of the bit is directly switched to a state that is dependent on the polarity of both the applied field pulses.
In order to improve memory density at a larger bit size, multi-state, multi-layer magnetic memory cells with magnetically coupled magnetic layers have been developed. See for example, U.S. Pat. Nos. 5,953,248 and 5,930,164, which disclose a antiferromagnetically coupled multi-layer structure having first and second magnetoresistive layers with a non-magnetic conducting layer situated in parallel juxtaposition between the pair of magnetoresistive layers. The pair of magnetoresistive layers in the antiferromagnetically coupled multi-layer structure are constructed to switch at different magnetic fields, by having different thicknesses or different magnetic material. Also, the pair of magnetoresistive layers in the antiferromagnetically coupled multi-layer structure each have a magnetic vector which are anti-parallel with no applied magnetic field due to the antiferromagnetic coupling of the pair of layers and the aspect ratio. The cell further includes a magnetoresistive structure having a magnetic vector with a fixed relationship to the vector of the second magnetoresistive layer. Electrically insulating material is situated in parallel juxtaposition between the antiferromagnetically coupled multi-layer structure and the magnetoresistive structure to form a magnetic tunneling junction.